ui: Show git-pull-request icon for closed pull request

[a1012112796]

As title, I think it's more reasonable, and it's the same as GH.

example view:

[codecov-commenter]

Codecov Report

Merging #12742 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #12742      +/-   ##
==========================================
- Coverage   43.40%   43.39%   -0.02%     
==========================================
  Files         648      648              
  Lines       71581    71581              
==========================================
- Hits        31072    31060      -12     
- Misses      35482    35492      +10     
- Partials     5027     5029       +2     

Impacted Files	Coverage Δ
modules/notification/mail/mail.go	34.48% <0.00%> (-3.45%)	⬇️
modules/notification/ui/ui.go	80.55% <0.00%> (-2.78%)	⬇️
services/pull/check.go	47.69% <0.00%> (-2.31%)	⬇️
modules/log/event.go	57.54% <0.00%> (-1.89%)	⬇️
models/notification.go	66.13% <0.00%> (-0.92%)	⬇️
models/error.go	34.81% <0.00%> (-0.86%)	⬇️
services/pull/pull.go	41.57% <0.00%> (-0.47%)	⬇️
modules/git/repo.go	49.74% <0.00%> (+0.50%)	⬆️
modules/git/utils.go	77.04% <0.00%> (+3.27%)	⬆️
modules/indexer/stats/db.go	60.86% <0.00%> (+17.39%)	⬆️
... and 1 more

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[6543]

@a1012112796 it's always nice to have screenshots ;)